At this place we may for a little while consider a few fundamental principles of construction whereby inventors have economized material, labor and energy by making their devices self-acting, and by so poising a contrivance that a mere touch at the right time and place sets it going. Steam Engines. Humphrey Potter was a boy whose duty obliged him to open and shut the valves of a Newcomen steam-engine as it slowly went its rounds. He was a human sort of boy, who liked play better than his irksome task, so he found a way to rid himself of the drudgery of constantly moving his valve-handles to and fro. He tied a rod to the walking beam in such wise that it opened the valve at the proper moment, and, at another point in its circuit, when necessary, closed it. Then and only then did the steam-engine become self-acting. In the best modern types of engine this automaticity goes far indeed. Not only does the mechanism pump water as required into both the boiler and the condenser, it shuts off steam instantly when the engine moves too swiftly, and, when the engine speed is sluggish the port betwixt boiler and cylinders is opened to the full. And further: automatic stokers bear coal into the furnace at a rate which varies with the demand, should the steam pressure fall through an undue call for power, then an extra quantity of coal is borne upon the grate-bars. When oil is the fuel automatic stoking is, of course, at its best, there being neither cinders nor ashes to be removed--a duty, by the way, which in large central stations requires extensive machinery, all automatic. Self-winding Clocks. The essence of automaticity is that mechanism at a certain, predetermined point in an operation shall perform a required act. Thus, to take the common example of a striking clock: at the end of each hour a detent is pulled so as to release a hammer which hits a gong the proper number of times. Let us suppose the clock to be driven by a weight or a spring in the ordinary way; every day or every week the weight or spring will require to be wound up. In time-pieces of a new variety the period during which no attention whatever is needed is lengthened to a year. The Self-winding Clock Company, of Brooklyn, New York, makes a clock which is driven by a fine spring, much like a common clock; that spring every hour is automatically wound up by a tiny electric motor connected with a small battery in the clock case. An attachment is provided by which, through the wires of the Western Union Telegraph Company, the clock is every hour regulated to the standard time of the National Observatory at Washington. The charge for this service is one dollar a month. [Illustration: Stop-motion.] Looms and Presses.